The dead load moment at the point of splice is X 1,210 X 172 = 329,000 ft.-lb. 173 The maximum live load shear will occur with the uniform load covering the girder up to the point of splice, and is = VL 1,040 X 32 X 16 = 10,850 lb. 49 The maximum live load moment occurs with the entire span loaded, and is ML = X 1,040 X 49 X 17 The total moment at the section is 611,000 ft.-lb. The amount of the moment carried by the web is equal to the ratio of the web area counted as flange area, to the total area of one flange and, considering the tension flange is 2.II The type of splice shown in Fig. 12 will be used. The number of shop rivets required by shear alone is determined by bearing on the in. web and is 19,850 ÷ 5,630 = 4. This number must be materially increased to take care of the moment. There should be two rows of rivets on each side of the splice. The arrangement shown in Fig. 12 represents the minimum number of rivets which can be conveniently used in this case Ed2 = 2 X (2)2(1 + 22 + 32 + 42 + 52 + 62 + 72 + 82) 2 Allowable bearing (Table 33, Appendix III) = 5,630 lb. This splice will be used as there is no place where it is advisable to reduce the number of rivets. The flange angles and cover plates can be obtained long enough for the entire span, so they will not have to be spliced. 9. DESIGN OF END BEARINGS.-Roller bearings are not required for spans less than 70 ft., so sliding bearings will be used for this girder. Slotted holes will be used in the sole plates to allow for a movement of at least in. at one end. The area of the wall plate must be at least where V the maximum end reaction, and ƒ = allowable bearing on concrete abutments. The sizes of the sole plates and masonry plates will probably be determined by the detail adopted. The thickness of the sole plate will be taken as § in. and the masonry plate as in. The anchor bolts will be hacked bolts, 1 in. in diameter and I ft. 3 in. long. CHAPTER XIII. DESIGN OF Low TRUSS HIGHWAY BRIDGES. Introduction.-Low truss highway bridges are used for spans of from 30 to 80 feet, and for special designs to 100 feet. The trusses may have either pin-connected or riveted joints. The trusses may have either half-hip, as in Fig. I or full slopes, as in Fig. 2, and may be either of the Warren type, as in Fig. 1, or of the Pratt type, as in Fig. 2. The cost is practically the same for the two types. Low truss highway bridges should always be made with riveted connections. Design of Riveted Trusses.-The author's specifications for the Design of Steel Highway Bridges, Appendix I, contain the following requirements for the design of low truss bridges. 83. Types of Truss.-Spans from 30 ft. to 80 ft.-Riveted plate girders or riveted low trusses for classes A, B, E1, E2 and E3, and riveted low trusses for classes C, D, and D. $54. Low Trusses.-Riveted low trusses shall have top chords composed of double web members with cover plate. The top chord shall be stayed against lateral bending by means of brackets or knee braces rigidly connected to the floorbeams at intervals not greater than twelve times the width of the cover plate. The posts shall be solid web members. The floorbeams shall be riveted, preferably above the lower chord. Pin-connected low truss bridges shall not be used. General Specification for Steel Highway Bridges adopted 1918 by the Engineering Institute of Canada contains the following specifications with reference to the design of low truss highway bridges. "Pony-truss bridges shall be of riveted type. Spans of 50 feet, center to center of bearings, or less, may have single-webbed trusses with T-chords; but all spans over 50 feet in length shall have double-webbed chords, and latticed or otherwise effectually stiffened web-members, unless otherwise specified by the engineer. "In all pony-truss bridges, the floor beams shall be rigidly connected to vertical truss-members; and stiffening gussets, as large as prácticable without interfering with the roadway clearances, shall be provided. The vertical truss-members and the floorbeam connections thereto shall, when practicable, be proportioned to resist, at the specified unit-stresses, a lateral force applied at the top-chord of the truss, equal to 2 per cent of the maximum top-chord stress. When im FIG. I. -4%6 Wheel Guard -446 Spiking Strip Floor Beam Cross Section. Plan LOW WARREN RIVETED HIGHWAY BRIDGE. GILLETTE-HERZOG MFG. CO. |
